1. Field of the Invention
This invention relates to an improved method and apparatus for controlling a fluid catalytic cracking system. More specifically, the present invention includes, as an element for control of the catalyst section, variable recycle of regenerated to spent catalyst in the regenerator.
2. Description of the Prior Art
Fluid catalytic cracking of petroleum fractions is a well-established refinery operation. The catalytic cracking apparatus per se comprises a catalyst section which is subdivided into a reactor section where catalytic cracking occurs, coupled with a regenerator section where coke deposited on spent catalyst is burned. The process operates essentially as follows. Fresh feed, usually preheated, mixed with catalyst, undergoes cracking within the reactor section. Products are removed from the reactor in the vapor phase and passed to a products recovery section comprising at least one main fractionator or distillation column for separation of the products into desired fractions. Spent catalyst, which has been coked by the cracking reaction, is continuously passed from the reactor to the regenerator by a spent catalyst transfer line. In the regenerator, the coke is burned by contact with an oxygen containing gas. Flue gas is passed from the regenerator, and regenerated catalyst is recirculated to the reactor via a standpipe where it is picked by the fresh feed hydrocarbon charge stream. The catalyst itself is finely divided and simulates a fluid in various portions of the catalyst section, whence the name of the process. In a typical operation, heat generated in the regenerator is carried by the hot regenerated catalyst to the reactor to supply heat for the cracking reactions. Typical fluid catalyst cracking systems are disclosed in U.S. Pat. Nos. 3,206,393 and 3,261,777.
The fluid catalytic cracking process has been improved in efficiency over the years. In particular, the discovery of zeolite catalysts with their greater activity and reduced coke make, and improvements in design of the reactor section to emphasize riser-cracking, are cases in point.
A recent innovation in regenerator section technology promises simultaneously to simplify the apparatus, more effectively utilize the heat evolved in regeneration, contribute better product quality, and reduce the inventory of catalyst. Whereas the conventional regenerator technology produces regenerated catalyst with about 0.3 wt.% residual coke, and a flue gas rich in carbon monoxide, the innovative technology produces regenerated catalyst substantially free of coke, i.e. less than about 0.08 wt.% and a flue gas in which the CO (carbon monoxide) has been substantially all burned to CO.sub.2 (carbon dioxide), i.e. the flue gas has a CO content less than about 4000 ppm and preferably less than about 2000 ppm (As used herein, ppm refers to parts per million parts by volume.) With the innovative technology, the need disappears for the CO-boiler plant conventionally used to complete the CO burning and recover sensible heat, because complete CO-burning takes place in the regenerator itself. Aside from simplified apparatus, other advantages accrue from the innovation which include: more active regenerated catalyst; better gasoline quality; more efficient transfer to the reactor section of the heat generated in coke burnoff; and a flue gas composition ecologically suitable for direct discharge to the atmosphere. For convenience, the conventional fluid catalytic cracking technology, in which the flue gas from the regenerator section contains substantially more than about 4000 p.p.m. of CO, will be characterized herein as operating in the "partial CO-burning mode." The process operating according to the innovative technology, in which the CO content of the flue gas is about 4000 p.p.m. or less, will be referred to herein as operating in the "complete CO-burning mode."
In spite of the significant advantages of the innovative technology, its widespread acceptance by the petroleum industry is questionable. For example, relatively small disturbances in the feedstock could cause premature failure of the cyclones used in the regenerator to disengage regenerated catalyst from flue gas. Also, there is very serious question that available control systems can make the catalytic cracking apparatus sufficiently adaptive to accept changes of feedstock quality without major or catastrophic upsets such as would lead to shutdown of the plant.
The problems that are encountered with control of fluid catalytic systems operating in the complete CO-burning mode may, at first glance, appear to be related to the unusually high temperatures that presently obtain in the regenerator. However, it will be shown that a serious, more subtle constraint is present.